A manufacturing facility reported that a Mitsubishi FG25K internal combustion forklift would violently stall whenever the operator attempted to lift a maximum capacity load. The machine idled perfectly and drove smoothly around the yard, but the moment the hoist lever was pulled with a heavy pallet on the forks, the engine bogged down and died. The maintenance team had already replaced the fuel filter and cleaned the carburetor, but the stalling persisted.
Because the stall only occurred under a combined load (engine torque for hydraulics), the initial thought was a hydraulic pump seizure placing excessive parasitic drag on the engine. A pressure gauge was installed on the main hydraulic relief valve. When the hoist was activated, the pressure spiked to 2200 psi (normal), but the engine rpm instantly plummeted from 2400 to 400 before stalling. The pump was not seizing; the engine was simply running out of torque and fuel.
The focus shifted to the fuel delivery system. The forklift utilizes a mechanical lift pump bolted directly to the engine block, driven by an eccentric lobe on the engine's camshaft. A clear hose was installed between the fuel filter and the carburetor inlet. At idle, the hose filled with fuel. However, when the hoist lever was pulled, the fuel flow sputtered, aerated, and the carburetor bowl ran dry within seconds, causing the stall. This confirmed a massive fuel starvation issue under demand.
The mechanic removed the mechanical lift pump. A manual actuation test showed the pump was moving fuel, but the lever spring felt incredibly weak. Upon disassembling the lift pump, the internal diaphragm was intact, but the actuating lever-which rests against the camshaft eccentric-was severely worn. More critically, a heavy, oily sludge was packed inside the pump's pulse dampening chamber. The sludge was causing the diaphragm to operate sluggishly, unable to keep up with the high fuel demand of the engine under hydraulic load.
To find the root cause of the sludge, the mechanic pulled the crankshaft pulley to inspect the timing cover. The bolt holding the camshaft eccentric lobe was slightly loose. The woodruff key securing the eccentric lobe to the camshaft snout had sheared in half. The lobe was spinning intermittently on the shaft, delivering erratic, half-strokes to the fuel pump lever. The loose lobe had also thrown off the ignition timing slightly, robbing the engine of the low-end torque required to run the hydraulic pump. The oily sludge in the pump was caused by a weeping front main seal, which the loose lobe had aggravated, allowing engine oil to be ingested into the lift pump's crankcase vent.
The repair required removing the front timing cover to replace the sheared woodruff key and torque the camshaft eccentric bolt to specification. A new mechanical lift pump was installed, and the weeping front main seal was replaced. After bleeding the fuel lines, the forklift lifted a 5,000-pound load without a single hiccup, restoring full operational capacity to the shipping dock.
